Integrated TIR prism and lens element

ABSTRACT

Disclosed is an optical component, which comprises a prism element adjacent to a lens element, where the two elements are separated by a small air gap. In disclosed embodiments, the elements have adjacent and parallel surfaces which are substantially planar and which, with the small air gap, operate through Total Internal Reflection (“TIR”) to direct light beams that strike the planar surfaces. Light beams that strike at less than the critical angle are internally reflected, while light beams which strike at greater than the critical angle pass through. The TIR surfaces thereby separate the desired optical signals from the spurious ones. The combined TIR prism lens operates as a single and integrated component which directs desired light beams to a reflective optical processing element such as a Spatial Light Modulator and which focuses the processed light beams as they leave the combined TIR prism lens.

TECHNICAL FIELD

Reflective optical systems with angular separation of illumination pathand reflection path.

BACKGROUND

In systems that employ a reflective optical device for the processing oflight there is typically a path through which the illumination lightbeam travels and a path through which the reflective light beam travels.These respective paths are referred to as the illumination path and thereflection path. When lenses or other optical elements are positioned toreceive the reflective light beam, these elements must be arrangedrelative to the illumination path so that they do not interfere with theincoming illumination beam. For example, in an optical projectionsystem, a Total Internal Reflection (“TIR”) prism may be used toseparate the illumination path from the reflection path. The reflectionpath would be referred to in this embodiment as a “projection path.” TheTIR prism may formed of two prism elements adjacent to each other, witha small air gap between them.

In one embodiment, the illumination beam is reflected at the air gap ofthe TIR prism using TIR reflection and is thereby directed towards thereflective optical device, which may be, for example, a Spatial LightModulator (“SLM”), which modulates the illumination light and provides amodulated, reflected light beam that may carry an image, data, or othersignal modulated on the light beam. This modulated, reflected beam isdirected at a high incident angle such that it passes through the TIRsurface of the TIR prism and through the air gap therein. If the lightbeam hits the TIR surface at less than the critical angle, it isreflected. This principle is used to reflect away the unwanted lightcomponents, such as from off-state pixels of the SLM or from otherstructures and surfaces of the SLM or other optical components in thesystem that are reflecting the light at angles outside of the desiredprojection angle. The light that passes straight through the TIR prismafter reflection by the SLM remains on the reflection or projectionpath, and is typically passed through a lens or through other opticalelements. The lens or other optical elements may, for example, projectthe light beams onto a display surface or direct them to other lenses orother optical elements.

One projection system is the Digital Light Processing (“DLP”) projectionsystem manufactured by Texas Instruments for use by display andprojector manufacturers. The optical processing elements in the DLPprojection systems are referred to as Digital Micromirror Devices(“DMD”), and they comprise hundreds or thousands of individualreflective pixel elements which, depending on electrostatic forcesplaced on the individual pixel elements, reflect the illumination lighton to the projection path or to a separate “off-state” path. Theaforementioned TIR reflection surface is used to reflect off theseoff-state light reflections, as well as to reflect away light beamsoriginating from extraneous surfaces such as the DMD supportingstructures, glass windows, prisms or flat-state pixels, which are pixelsnot deflected at the ideal reflection angles. Once the desired lightbeams have been directed to the projection path, they are typicallyreceived by a lens, which focuses the beams either on to a projector orsubsequent optical elements in the projection path.

SUMMARY

Embodiments described in this patent application show the use of acombined TIR element or TIR prism and a lens, which can be combined toreduce the number of optical components in the system, reduce opticalpath length, and thereby increase ease of manufacturing and reducecosts. In approaches described herein, prisms are mounted to lenselements with a small air gap between them to form a combined TIR lenselement. The air gap which separates the adjoining, parallel plane facesof the two sub-elements of the combined TIR lens element—the TIR elementand the lens element—is on the order of 1 mm and generally within therange of a few wavelengths to 2–3 mm.

The combined TIR lens element comprising a TIR prism element and a lenselement performs the functions of: (a) separating the illumination beamfrom the reflection or projection beam; (b) separating the off-state,flat-state, and spuriously scattered light beams from the intendedreflection or projection beam after they are reflected from the SLM; and(c) optical manipulation of the beam. Using the approaches described inthis specification, the elements that perform these disparate functionscan be integrated into a single combined element, which is describedherein as a combined TIR lens element. In a given application, it is notnecessary that all these functions be performed by the combined TIR lenselement, as the embodiments described herein can be employed toaccomplish some or all of the described functions.

Using approaches described in this specification, it is possible for thelens sub-element of the combined TIR lens element to direct anillumination beam onto an SLM without first relying on a TIR reflectionto reflect the beam onto the SLM. Alternatively, the illumination beammay enter the combined TIR lens element from a side angle (i.e., roughlyparallel to the face of the SLM) and then rely on TIR reflection for itsdirection to the SLM surface. The embodiments described in thisspecification employ a combined TIR lens element both to separate eitherthe illumination and projection light bundles or the projection andoff-state light bundles from each other and to focus or “power” thereflected or projection light beam. Although, this specification refersprimarily to optical processing in the context of a projection system,it could also be employed in data communications applications, forinstance, which use reflective optical light switching or in other typesof optical data or image processing, switches, or transmission. Stillother optical processing applications can gainfully use the techniquesand structures described in this specification. These other approachesare encompassed within the scope of the claims set forth in thisapplication, and the scope of the claims should not be limited to thespecific light projection embodiments described in the detaileddescription of the embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing of a prior art optical projection system which usesspatially separated elements for TIR reflection and lens powering;

FIG. 2 is a drawing of an embodiment that uses a combined TIR lenselement that comprises a TIR prism and a lens in a telecentricprojection application;

FIG. 3 is a drawing of an embodiment that uses a combined TIR lenselement that comprises a TIR prism and a lens in a non-telecentricprojection application;

FIG. 4 is a drawing of a combined TIR lens element that employs twolenses having adjacent planar faces that act as TIR surfaces and alsohave convex lens surfaces in an application that uses the focal surfacesof the lenses to direct an illumination beam to a reflecting element;

FIG. 5 is a drawing of a combined TIR lens element that employs twolenses having adjacent planar faces that act as TIR surfaces and alsohave convex lens surfaces in an application which uses TIR reflectionfrom a planar surface of a lens to direct an illumination light beam toa reflecting element;

FIG. 6 is an exploded-view drawing of a three-dimensional combined TIRlens element, which comprises a TIR prism with a conical cavity and alens with a conical outer surface that mates with the conical cavity ofthe TIR prism element;

FIG. 7 is a side-view drawing of the three-dimensional combined TIR lenselement of FIG. 6; and

FIG. 8 is a drawing of a packaged SLM module having an integrated,combined TIR lens element.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows a prior art optical projection system 100. Shown in FIG. 1is a light source 102, which provides a white light for the projectionsystem. Also shown is a lens 104, which focuses the light for passagethrough a color wheel 106 . The color wheel 106 spins and alternativelyfilters the white light into its red, green, and blue components. Bysynchronizing the operation of a Spatial Light Modulator (“SLM”) 120,which is the active element in the projector path, with the rotation ofthe three colors of the color wheel 104, it is possible to project afull-color image from the system. In essence, the image would be aquickly alternating set of red, green, and blue images, but theflickering of the images are at a speed faster than the human eye isable to resolve, and accordingly the eye will “average” the colors andsee a full-color image.

After passing through the color wheel 106, the light continues on theillumination path 108 through a lens 110. The lens 110 is typically agroup of lenses and mirrors which together focus and direct light fromthe light source 102 into the TIR prism unit 112. In the embodimentshown, the TIR prism unit 112 is comprised of two TIR prism elements 112a–b, which are separated by a small air gap 113. The differing index ofrefraction between the prism elements 112 a–b and the air in the air gap113 causes light striking at less than a critical angle at theprism-to-air-gap surface to be reflected according to principles ofTotal Internal Reflection (“TIR”). In the embodiment shown in FIG. 1,the illumination path 108 is directed toward the SLM 120 or otherreflecting element by a TIR reflection at point 121.

The SLM 120 receives the light beam along the illumination path 108 andreflects it onto the reflection or projection path 130. In oneembodiment, the SLM 120 is a Digital Micromirror Device (“DMD”), whichis an array of micromirrors that are is fabricated on a semiconductorsubstrate. These micromirrors are controlled through the application ofelectrostatic forces from underlying electrical control circuitry, whichserves to pivot the micromirrors into “on” or “off” positions as isshown in FIG. 1A illustrating “on” pixel 122 and “off” pixel 124. Theunderlying electrical control circuitry is typically fabricated in or onthe surface of the semiconductor substrate using standard integratedcircuit process flows. This circuitry typically includes, but is notlimited to, a memory cell associated with and typically underlying eachmirror and digital logic circuit to store the frame of digital imagedata or other information to be optically transmitted through modulationof the SLM pixels.

Typically, as shown in FIG. 1A, the micromirrors will pivot 10–12degrees in one direction for the mirrors' “off” state and 10–12 degreesin the other direction for the mirrors' “on” state (see pixels 124 and122, respectively). Thus, in an approach where the principal rays fromthe micromirror device 120 are reflected at a normal angle to thesurface of the device, the approach angle of the incoming light signalfrom the TIR reflection will be two times the pivot angle, or between 20and 24 degrees from normal. The TIR surface is preferable arranged toallow the reflected beam, which is on the reflection or projection path130 to pass directly through, in a direction that is approximatelynormal (at a right angle) to the face of the DMD or other SLM 120. The“off” pixels will leave the surface of the DMD 120 at an angle of 40 to48 degrees, which comes from the original 20 to 24 degree approach addedto the 20 to 24 degree of rotation from an “on” state pixel to an “off”state pixel. The TIR surface of the TIR prism element 112 a ispreferably configured to reflect away, using TIR, these “off-state”beams from the reflection or projection path 130 so the beams do notinterfere with the desired image projection or optical data reflection.The TIR prism unit 112 is also preferably arranged to reflect away the“flat state” reflections off of the SLM, which are the reflections fromthe various SLM surfaces and support structures, as well as, forinstance, reflections from micromirror pixels which may be in transitionand not at their fully on or fully off positions.

Provided in the reflection or projection path 130 is a lens 140, whichfor illustration purposes is shown as a single lens, although it couldbe a group of lenses. The lens 140 might be a projection lens thatfocuses images on a display screen, or it could be another type ofoptical lens for manipulating the optical signal for transmissionthrough, for example, an optical communication system.

There are several challenges posed by the use of the prior-art system ofFIG. 1. One challenge is to separate the illumination path and thereflection/projection paths so that the optical elements (such as theprojection lens 140) do not interfere with the light beams on theillumination path. In the system shown in FIG. 1, this separation isaccomplished by sending the incoming illumination light bundles along anillumination path 108 that is essentially parallel to the surface of theSLM 120 and by then reflecting the beam using the TIR surface of the TIRprism unit 112. Another challenge is to make the total assembly 100 assmall as possible. Another challenge is to manufacture the system asefficiently as possible and using the least expensive assembly ofcomponents.

FIG. 2 is a drawing of a combined TIR lens element 200 that comprises alens element 200 a and a TIR prism element 200 b in a telecentric imageprojection application. This combined TIR lens element 200 could beused, for example, in the application of FIG. 1, as a replacementelement for both the TIR prism unit 112 and the lens 140. In thisembodiment, rather than using a TIR prism unit to direct theillumination path 108 onto the SLM 120, the curved face 202 a of thelens element 200 a is used to bend the relatively shallow-approachingillumination beam into a steeper angle of incidence to the SLM 120. Theshallow approach angle also facilitates the placement of opticalelements in the reflection/projection path 130 and closer to thereflective element 120 without those optical elements interfering withthe illumination beam 108. As before, the SLM 120 modulates theillumination beam with image information (for display) or other opticaldata information (e.g., for data communication). As shown in FIG. 2, themain beam in this embodiment is reflected approximately normally fromthe surface of the SLM 120 and has an incident angle to the TIR surface202 b of the TIR prism element 200 b that is greater than the criticalangle for that surface. In a communication system in which the SLMelement 120 is used as an optical router or switch, the off-state beamwould ideally be reflected using TIR reflection to a defined alternatesignal path 150. In a system in which all off-state light bundles arespurious, such as in a projection system, then the alternate signal path150 would be directed away from the system and in some instances to astructure that is operable to absorb the light energy in a way thatprotects the optical components. In either type of system, the lenssurface 203 of the lens element 200 a can be used to focus or otherwisemanipulate the reflected/projection light beam 130 as it exits thecombined TIR lens element 200. The combination of these functions in asingle integrated element allows optical system designers to design morecompact and cheaper optical systems.

The combined TIR lens element 200 may be formed by joining two opticalpieces together, one piece forming the lens element 200 a and the otherforming the TIR prism element 200 b. The pieces are separated to formthe air gap 213 through the use of spacers 205. Alternatively, thecombined TIR lens element 200 may be formed by taking a single opticalpiece and forming the air gap 213 as a slit in that single piece, wherethe air gap slit 213 separates the lens element 200 a of the piece fromthe TIR prism element 200 b. These same assembly and/or manufacturingtechniques can also be used in the embodiments described below. Othermeans of manufacturing combined TIR lens elements according to this orother embodiments are also possible.

FIG. 3 is a diagram of a system which is essentially similar to thesystem of FIG. 2, but which applies the combined TIR lens element 200 ina non-telecentric application. In this approach, thereflection/projection beam 130 does not reflect from the SLM surface atan angle normal to the surface, but instead reflects away at an angle αrelative to normal. Operating the system in this non-telecentricconfiguration allows for a greater angular separation between theillumination path 108 and the reflection/projection path 130. To operatethe system in this configuration, the TIR surface 202 b is oriented suchthat, as with the telecentric configuration, the primary ray from theSLM surface hits the TIR surface at greater than the critical angle andthereby passes through the TIR surface 202 b and on out through the lenselement 200 b. The lens element 200 b is designed to focus or otherwisemanipulate the reflective/projection light bundles. As with FIG. 2, thisdesign results in improved compactness of the system, as well as in animproved component and assembly cost.

FIG. 4 is a drawing of a combined TIR lens element 400 that is formed oftwo lenses 400 a–bhaving adjacent planar faces 402 a–b, separated by anair gap 413, which act as TIR surfaces. As with the embodiments of FIGS.2–3, this structure may be configured for use in either telecentric ornon-telecentric applications by proper design and alignment of theoptical elements in the system. The lens elements 400 a–b also haveconvex lens surfaces 403 a–b that focus or otherwise manipulate thelight beams passing through the lenses. The use of the second lens 400b, rather than a TIR prism element, allows for two identical componentsto be used to form the combined TIR lens element 400, rather than twodistinct piece parts (i.e. the TIR prism element and the lens element).The use of identical components is optional, and allows for a moreefficient procurement of components for the final assembly.Non-identical lenses could also be used in certain embodiments.

FIG. 5 is a drawing of an embodiment that essentially uses the same typeof combined TIR lens element 400 as was used in FIG. 4. As in FIG. 4,these lenses have adjacent planar faces 402 a–b, which act as TIRsurfaces and also have convex lens surfaces 403 a–b. In this embodiment,however, the illumination path 408 approaches the combined TIR lenselement 400 from the side. As in the FIG. 3 embodiment, a TIR reflectionis used to direct the light beam over the illumination path 408 onto theSLM 120. The lens surfaces (403 a–b) of both lenses 400 a–b can again beused to focus or otherwise manipulate the light beams as they exit andre-enter the combined TIR lens element 400 over both the illuminationpath 408 and the reflection or projection path 430. This embodiment canalso be employed in both telecentric and non-telecentric applications,depending on whether the principal ray from the SLM 120 is reflectedalong the normal axis to the surface or at an off-normal angle. Thefocusing and alignment of the lenses will be different in thetelecentric and non-telecentric applications, but the functions of thevarious parts of the combined TIR lens element of FIG. 5 are generallythe same as described in FIG. 4.

FIG. 6 is an exploded view drawing of a three-dimensional combined TIRlens element 600, which comprises a lens element 600 a with a conicalouter surface 602 a and a TIR prism element 600 b with a conical cavity602 b. The conical cavity 602 b mates with the conical outer surface 602a, and as with the two-dimensional applications described above, spacers605 would ideally be used to create a small air gap (not shown, see FIG.7) whereby there is established an index of refraction differencebetween the lens element and the air and the prism element and the air,and further whereby a light beam approaching the air-gap interface atless than the critical angle is totally internally reflected within therespect lens or prism in which the light beam is traveling. Both thelens element 600 a and the TIR prism element 600 b are generallycylindrical in shape and are circularly symmetrical about the axis oftheir respective cylinders.

FIG. 7 is a side view drawing of the embodiment of FIG. 6. As mentioned,this approach uses a circular lens element 600 a, which has a conicalprojection 602 a opposite the lens surface 603. The lens surface 603 is,in other words, at one end of the cylindrical form of element 600 a,whereas the conical projection 602 a is at the other end of thecylinder. The other component of the three-dimensional combined TIR lenselement 600 is a TIR prism element 600 b. The TIR prism element 600 b isalso generally circularly symmetrical about its cylindrical axis, havinga generally planar surface that is opposite to its conical indentation602 b. Because of the three-dimensional nature of this combined TIR lenselement 600, approaching light bundles about a 360 degree circlerelative to the cylindrical axes of these elements 600 a–b can bedirected toward the SLM 120 or other reflective element.

The illumination path 708, in the example of FIG. 7, is essentially adisk-shaped path over which light bundles approach the TIR surface 602 bfrom concentric positions about the center of the lens. These lightbundles are directed toward the reflective element 120 by reflection offof the TIR surface 602 b. The beams are reflected, and optionally,modulated, by the SLM 120 or other reflective surface, and then proceedover reflected/projection light path 730. As before, light bundles thatare reflected off of the SLM 120 or other reflective surface at anglesless than the critical angle (such as, for instance, if the light isreflected from “off” pixels of the SLM) are reflected away by the TIRsurface 602 b and accordingly do not interfere with the intendedreflected or projected light beams travelling on the reflective path130.

FIG. 8 is a drawing of an integrated SLM module onto which a combinedTIR lens element 800 is mounted. Integrated SLM modules are commonlysold packaged in ceramic with a clear window mounted above them throughwhich light can pass. In the embodiment shown in FIG. 8, even furtherspace savings can be realized in an optical system design by eithermounting a combined TIR lens element 800 directly onto such a glasswindow or by replacing the package window with the TIR lens elementitself. FIG. 8 illustrates in a partial assembly view how such anintegrated SLM module including a combined TIR lens element 800 could beimplemented. As described previously, the combined TIR lens elementcomprises a TIR lens element 800 aand a TIR prism element 800 b, whichtogether serve both to direct incoming light bundles to the spatiallight modulator 120 and to focus the reflected bundles as they leave thecombined TIR lens element 800.

Although some of the embodiments have been described above in thetwo-dimensional sense, the applications described in the two-dimensionalapproaches could also be employed in three-dimensional applications.Such three-dimensional applications include, but are not limited tomicroscopes and telescopes.

Additionally, although embodiments have been described above forapplication in SLM projection systems, the advantages gained by use ofthe combined TIR lens element can be applied in many fields employingreflective optics. For example, the described embodiments could beemployed for printing applications, optical communication applications,and others. The light beams described in the embodiments above are ofwhite (full-spectrum) light, as well as beams filled from the white. Thedescribed embodiments can also be with single wavelength light beamssuch as generated from the laser light or multiple single wavelengthbeams generated from multiple lasers and/or wavelength-divisionmultiplexers.

A few preferred embodiments have been described in detail hereinabove.It is to be understood that the scope of the invention also comprehendsembodiments different from those described, yet within the scope of theclaims. Words of inclusion are to be interpreted as nonexhaustive inconsidering the scope of the invention. While this invention has beendescribed with reference to illustrative embodiments, this descriptionis not intended to be construed in a limiting sense. Variousmodifications and combinations of the illustrative embodiments, as wellas other embodiments of the invention, will be apparent to personsskilled in the art upon reference to the description. It is thereforeintended that the appended claims encompass any such modifications orembodiments.

1. An optical assembly comprising: a first element having a generallycylindrical shape, a generally circular base, and a conical indentationwithin the cylindrical shape that serves as a total internal reflectionelement for light from the perimeter of the cylindrical shape incidentthe conical indentation at greater than the critical angle; a lenselement having a surface adjacent and substantially parallel to thesurface of the conical indentation of the first element, the lenselement also having a curved surface for focusing light passing throughsaid curved surface; and a reflective member positioned adjacent thebase of the first element to reflect light from the conical indentationof the first element to the curved surface of the lens element.
 2. Theoptical assembly of claim 1 wherein the lens element is circularlysymmetrical, having a generally cylindrical shape but having a conicalend which mates with the conical indentation in the first element andhaving a lens surface opposite the conical end.
 3. The optical assemblyof claim 1 wherein the end of the generally cylindrical shape of thefirst element that is opposite to the conical indentation issubstantially planar.
 4. The optical assembly of claim 1 wherein the endof the first element that is opposite to the conical indentation issubstantially curved to form a lens surface.
 5. The optical assembly ofclaim 1 wherein the reflective member is a micromirror array.